To me, 2020 belongs to Thiem. He was very close in Melbourne and finally had his breakthrough in New York. Medvedev had two good last tournaments (Paris-Bercy and London), but before that he did not make a single final, so it wasn't his best year.. And the main takeaway from London is that both Thiem and Medvedev managed to beat Nadal and Djokovic each. I hope 2021 will be the year when the generation change finally happens, and maybe Thiem will at last succeed in beating Nadal in Paris.
On the women's side the most impressive figure was Iga Świątek, and her demolition of Halep was probably the best women's match of the year. I hope she won't end up a flash in the pan and we will see her again. Sonia was good too, probably the runner-up.